A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. For example, Pursifull et al. describe a system in US 2011/0132335 wherein compressed air is stored in a boost reservoir and is dispensed into the intake manifold when insufficient compressed air is available from the turbocharger compressor. In particular, the boost reservoir is charged with fresh intake air and/or effluent from one or more unfueled cylinders. By dispensing extra compressed air from the boost reservoir to the intake manifold, torque corresponding to the dispensed air can be provided to meet the torque demand while the turbine spins up.
However, the inventors herein have identified potential issues with such a system. As an example, during selected boost conditions, such as when an operator tips-in while the boost level is above a threshold, EGR may be requested but not available as rapidly as required. Specifically, recirculated exhaust gas may not be immediately available via low pressure EGR due to the slower response time of the low pressure EGR. Recirculated exhaust gas may also not be immediately available via high pressure EGR due to the pressure difference between the intake and exhaust manifolds which would cause the high pressure EGR to flow backwards into the exhaust manifold. Consequently, increased EGR may not be possible and engine performance during those selected boosted conditions may be degraded.
Some of the above issues may be at least partially addressed by a method for a turbocharged engine. In one embodiment, the method comprises, during lower boost conditions, charging a boost reservoir with at least combusted exhaust gas to a first pressure; during higher boost conditions, raising the boost reservoir to a second, higher pressure by further charging the boost reservoir with compressed intake gas; and then discharging the pressurized gas from the boost reservoir to an intake manifold. In this way, exhaust gas may be pre-stored in a boost reservoir for subsequent recirculation, and further can be increased to a sufficient pressure (even above the highest exhaust pressure) so that sufficient exhaust gas can be delivered to the intake when desired (even when the intake is highly boosted).
For example, during boosted engine operation, while the boost level is below a threshold, a boost reservoir may be charged with at least some exhaust gas at lower pressure from the exhaust manifold, thus providing a source of stored EGR. In response to a tip-in (e.g., towards wide open throttle), while the boost level is above the threshold, high pressure EGR may be immediately requested. To provide the higher pressure EGR, compressed intake gasses can be added to the boost tank to raise the pressure of exhaust gasses already stored therein. In this way, even though the exhaust pressure may not be high enough to charge the tank with exhaust gas at sufficient pressure for delivery to the intake, the addition of higher pressure intake gasses can raise the pressure, thus allowing at least some exhaust gas to be delivered to the intake, even when highly boosted. The discharged high pressure EGR may enable improved combustion control and reduced NOx emissions. In this way, combusted exhaust gas may be pre-stored in a boost reservoir to later supplement high pressure or low pressure EGR.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.